The Johnson & Johnson (Janssen) COVID-19 vaccine is a single-dose inoculation that uses an adenovirus, a common cold virus modified to be harmless, to deliver genetic instructions to the body’s cells. This viral vector technology prompts the immune system to recognize and defend against the SARS-CoV-2 virus. Like all medical interventions, this vaccine is associated with a range of side effects that have been closely monitored by health organizations since its introduction. These effects span from common, temporary reactions to extremely rare, but serious, adverse events that have guided regulatory actions.
Common and Temporary Reactions
The majority of individuals who received the Janssen vaccine experienced mild to moderate side effects, which typically resolved within one to three days following vaccination. These common reactions are considered normal signs that the body’s immune system is successfully responding to the vaccine. The most frequently reported localized symptoms were pain, redness, and swelling at the injection site. Systemic, or body-wide, reactions were also common, including general fatigue, headache, muscle aches, fever, and nausea.
Understanding Thrombosis with Thrombocytopenia Syndrome (TTS)
Thrombosis with Thrombocytopenia Syndrome (TTS) is a rare but serious adverse event characterized by the combination of blood clots (thrombosis) and low blood platelet counts (thrombocytopenia). This condition was a primary concern with the Janssen vaccine and similar adenovirus vector vaccines. The blood clots often occur in unusual locations, such as the cerebral venous sinuses in the brain or the abdominal veins.
The onset of TTS symptoms typically occurs within a specific timeframe, usually between six and 14 days after vaccination. The reported rate of TTS was approximately two cases per million doses administered, though this rate was higher in certain groups. Specifically, the risk appeared to be greatest in women under the age of 50.
Immediate medical attention is necessary if certain warning signs appear in the weeks following vaccination. These symptoms include severe or persistent headaches, blurred vision, shortness of breath, or chest pain. Other signs can involve leg swelling, persistent abdominal pain, or tiny blood spots under the skin beyond the injection site.
Guillain-Barré Syndrome (GBS) Association
Another very rare neurological condition associated with the Janssen vaccine is Guillain-Barré Syndrome (GBS). GBS is a disorder where the body’s immune system mistakenly attacks the peripheral nerve cells, causing muscle weakness and sometimes paralysis. While the exact cause is not fully understood, GBS is often triggered by infections.
The FDA added a warning about the increased risk of GBS to the vaccine’s fact sheets in July 2021. Symptoms typically began within 42 days of receiving the vaccination and often started with weakness or tingling sensations in the legs or arms that worsened and spread. This condition is distinct from TTS, involving different symptoms and affecting the nervous system rather than the blood clotting system.
The reported cases of GBS were extremely rare, with approximately 100 cases reported after 12.8 million doses of the Janssen vaccine had been administered. Symptoms requiring immediate medical care include difficulty walking, trouble with facial movements like speaking or swallowing, or double vision. The available data did not show a similar pattern of GBS association with the mRNA-based COVID-19 vaccines.
Current Regulatory Status and Usage
The risk of TTS led to a significant shift in the official guidance from regulatory bodies in the United States. Following a review of the data, the Centers for Disease Control and Prevention (CDC) expressed a clinical preference for the mRNA vaccines (Pfizer-BioNTech and Moderna) over the Janssen vaccine due to the rare but serious risk of TTS.
The Food and Drug Administration (FDA) subsequently revised the Emergency Use Authorization for the Janssen vaccine to limit its use. The vaccine is now generally reserved for adults aged 18 and older who either cannot receive an mRNA vaccine for medical reasons, such as a severe allergic reaction to a component, or for those who would otherwise remain unvaccinated. This guidance ensures that the Janssen vaccine remains an option for individuals who face barriers to accessing other vaccines or who specifically request it after receiving full risk counseling.